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Why does our planet have an atmosphere containing about 21 per cent oxygen and why do we depend critically on this fact to keep alive? It's not just that we breathe it; if there were no oxygen we would probably have no oceans and we would be bathed in lethal ultraviolet light. The fact that our nearest neighbours, Venus and Mars, have no oceans today may be because they never harboured oxygen-producing organisms. The Earth did, and this made all the difference.

That much is true, but the story is more complicated than this. Until recently the prevailing wisdom was that when oxygen appeared, thanks to photosynthesis by certain bacteria (cyanobacteria), it was toxic to most forms of life and this caused a mass extinction, an 'oxygen holocaust'. But some bacteria were able to adapt to oxygen and eventually to use it to produce energy, and the evolution of complex life was the result. Read more